The present disclosure relates to Direct Sequence Spread Spectrum (DSSS) wireless communications and communication devices and more particularly to a device and method for fast transition or substantially immediate transition from preamble synchronization to data demodulation in severe Doppler impairment wireless channels.
In DSSS wireless communications, a data packet being communicated between two devices comprises of a preamble portion and a payload data portion. The preamble portion serves to assist a receiver device to detect a (digitized) arrival data packet in baseband and to synchronize the arrival baseband data packet with the receiver's local Pseudorandom Noise (PN) sequence generators. In DSSS communications, payload data demodulation at the receiver may reconstruct original data only when the PN sequence generated from the receiver's local PN sequence generator precisely aligns with the identical copy of PN sequence embedded in the arrival baseband data packet. PN sequences are in the unit of chips. In DSSS communications, precise chip alignment is accomplished in two steps. In the first step, the preamble synchronization coarsely aligns local PN sequence with the PN sequence embedded in the arrival baseband data packet (may also be referred to as arrival PN sequence herein). The coarse alignment synchronizes the local and arrival PN sequences within a [−½, +½] chip interval. In the second step, a baseband Chip Tracking Loop (CTL) at receiver employs a feedback structure to fine tune the chip alignment between local and arrival PN sequences. The goal of fine tuning is to align the local and arrival PN sequences within a sub-chip interval. As an example, the sub-chip interval may be one-sixteenth of a chip. The fine tuning consists of an initial pull-in process to remove initial chip phase offsets between local and arrival PN sequences, and a subsequent baseband tracking process to track the baseband Doppler frequency drift between local and arrival PN sequences that may be caused by the relative moving velocity and the relative clock drift rate between transmitter and receiver devices.
Generally, the initial pulling process in the second-step synchronization incurs latency and causes communication bandwidth waste. This is because data demodulation with desirable bit error rate (BER) may only be achieved after chip phase offset between local and arrival PN sequences has been aligned within a substantially small fraction of a chip interval. The initial pulling latency is particularly significant in severe Doppler impairment of wireless channels, wherein the baseband CTL at the receiver has to pull and track a large amount of initial chip phase and baseband frequency offsets between local and arrival PN sequences. Severe Doppler impairment of wireless channels may be characterized by a clock drift between transmitter and receiver devices larger than several (for example about 5 or higher) Parts Per Million (ppm). One well-known solution to reduce the initial pulling latency is to over-sample the arrival baseband signal. Instead of using the Nyquist sampling rate at 2 samples per chip interval, a higher sampling rate, e.g., 4 or 8 samples per chip interval may be used. The high sample rate allows the preamble synchronization process to achieve coarse alignment at a sub-chip accuracy, for example, [−¼, +¼] or [−⅛, +⅛] chip interval, and allows baseband CTL to quickly remove an initial chip offset at a small faction of chip interval. This solution is effective in reducing the latency in initial pulling of the receiver's baseband CTL process. However, its reliance on a high sampling rate significantly increases the hardware implementation cost of the preamble synchronization structure.
Additionally, in DSSS communications systems and similar systems, the use of Offset Quadrature Phase Shift Keying (OQPSK) intended to remove the signal signature, may cause cross-talk between in-phase and quadrature components. The cross-talk in-phase and quadrature components may degrade demodulation performance, and it may be difficult to eliminate the cross-talk at low data spreading ratios.